This invention relates to liquid crystal materials, in particular it relates to liquid crystal materials which exhibit one or more smectic phases.
Liquid crystals can exist in various phases. In essence there are three different classes of liquid crystalline material, each possessing a characteristic molecular arrangement. These classes are nematic, cholesteric and smectic. A wide range of smectic phases exists, for example smectic A and smectic C. Some liquid crystal materials possess a number of liquid crystal phases on varying the temperature, others have just one phase. For example, a liquid crystal material may show the following phases on being cooled from the isotropic phase:-isotropic-nematic-smectic A- smectic C-solid. If a material is described as being smectic A then it means that the material possesses a smectic A phase over a useful working temperature range.
Materials possessing a smectic Axe2x80xa2(SAxe2x80xa2) phase may exhibit an electroclinic effect. The electroclinic effect was first described by S. Garoff and R. Meyer, Phys. Rev. Lett., 38, 848 (1977). An electroclinic device has also been described in UK patent application GB 2 244 566 A. This particular device helps to overcome the poor alignment problems of electroclinic (EC) devices using a surface alignment that gives a surface tilt within a small range of angles.
When a smectic A phase is composed of chiral molecules, it may exhibit an electroclinic effect, ie a direct coupling of molecular tilt to applied field. The origin of the electroclinic effect in a smectic Axe2x80xa2phase composed of chiral polar molecules has been described to Garoff and Meyer as follows. The application of an electric field parallel to the smectic layers of such a smectic Axe2x80xa2phase biases the free rotation of the transverse molecular dipoles and therefore produces a non-zero average of the transverse component of the molecular polarization. When such a dipole moment is present and coupled to the molecular chirality, a tilt of the long molecular axis (the director) is induced in a plane perpendicular to the dipole moment.
In thin samples for example 1-3 xcexcm and with the smectic layers tilted or perpendicular with respect to the glass plates the electroclinic effect is detectable at low applied fields.
In an aligned smectic A sample a tilt of the director is directly related to a tilt of the optic axis. The electroclinic effect results in a linear electro-optic response. The electro-optic effect can manifest itself as a modulation of the effective birefringence of the device.
Electroclinic devices are useful, for example, in spatial light modulators having an output that varies linearly with applied voltage. A further advantage of EC devices is that they have high speed response times, much faster than nematic based devices, including twisted nematic based devices. Unlike ferroelectric devices the EC device is not bistable.
The electroclinic effect is sometimes referred to as the soft-mode effect see G. Andersson et al in Appl. Phys. Lett., 51, 9, (1987). 
In general terms, regarding the electroclinic effect, it is advantageous if on applying a small voltage there results a large induced tilt.
An increase in induced tilt may result in an increase in contrast ratio or in phase modulation depth.
It is also advantageous if a large induced tilt can be obtained at as low a voltage as possible.
It is also advantageous if the relationship between molecular induced tilt and applied voltage is temperature independent. When an increase in applied voltage results in little or no change in induced tilt then the material being tested is generally referred to as exhibiting a saturation voltage effect.
Wand et al in Abstracts of the 4th International Conference of Ferroelectric Liquid Crystals Sep. 28-Oct. 1, 1993 in Tokyo, Japan: Paper P90, pages 257-58 and see references therein, report on novel electroclinic materials. They suggest that the reason for the materials they report exhibiting a large and temperature insensitive electroclinic tilt is due to the absence of a Cxe2x80xa2phase underlying the Axe2x80xa2phase and that it is usual for materials possessing a Cxe2x80xa2phase below the Axe2x80xa2phase to exhibit a relatively steep temperature dependence of induced tilt.
By SAxe2x80xa2is meant a SA phase which contains some proportion of chiral molecules.
According to this invention an electroclinic device comprises two spaced cell walls each bearing electrode structures and treated on at least one facing surface with an alignment layer, a layer of a smectic liquid crystal material enclosed between the cell walls, characterised in that th liquid crystal material contains one or more of the compounds described by formula I: 
wherein
X may be CN, NO2, CF3, CF2H, CFH2, halogen, hydrogen, alkyl or alkoxy;
X2 may be CN, NO2, CF3, CF2H, CFH2, halogen, hydrogen, alkyl or alkoxy;
X3 may be CN, NO2, CF3, CF2H, CFH2, halogen, alkyl, alkoxy or hydrogen;
X4 may be CN, NO2,CF3, CH2H, CFH2, halogen, alkyl, alkoxy or hydrogen;
provided that at least one of X or X2 is CF3, CF2H, CFH2, CN, NO2 or halogen;
A and B are independently phenyl, mono-fluorinated phenyl, di-fluorinated phenyl or cyclohexyl;
Y may be single bond, COO, OOC, C=C;
q may be 0 or 1;
R1 may be an end group of the following Formula II: 
wherein
Z may be a single bond, O, CO2, (CH2)n or (CH2)n) where n may be 1 or 2;
J and M may be independently H, C1-4 alkyl;
W may be C1-C12 straight or branched alkyl chain;
R2 may contain 1-14 carbon atoms wherein one or more non-adjacent CH2 groups may be replaced by CO2 or O.
Preferably:
W is C1-C8 straight chain alkyl, Z is O or CH2O, M or J are independently chosen from H or Me or Et;
R2 contains 5-12 carbon atoms wherein one of the CH2 groups may be replaced by CO2 or O;
q=1;
A and B are both phenyl;
X3 and X4 are both H;
Y is CO2 or a single bond;
X or X2 is NO2 or CF3 or halogen or CN.
According to a second aspect of this invention compounds of the following formula are provided: 
wherein
X may be CF3, CH2H, CFH2, hydrogen, NO2, alkyl or alkoxy;
X2 may be CF3, CF2H, CFH2, hydrogen, NO2, alkyl or alkoxy;
X3 may be CF3, CF2, CFH2, alkyl, NO2, alkoxy or hydrogen;
X4 may be CF3, CF2H, CFH2, alkyl, NO2, alkoxy or hydrogen;
provided that at least one of X or X2 is CF3, CF2H, CFH2, NO2;
A and B are independently phenyl, mono-fluorinated phenyl, di-fluorinated phenyl or cyclohexyl;
Y may be single bond, COO, OOC, C=C;
q may be 0 or 1;
R1 may be an end group of the following Formula II: 
wherein
Z may be a single bond, O, CO2, (CH2)n or (CH2)nO where n may be 1 or 2;
J and M may be independently H, C1-4 alkyl;
W may be C1-C12 straight or branched alkyl chain;
R2 may contain 1-14 carbon atoms wherein one or more non-adjacent CH2 groups may be replaced by CO2 or O.
Preferably:
W is C1-C8 straight chain alkyl, Z is O or CH2O, M or J are independently chosen from H or Me or Et;
R2 contains 5:12 carbon atoms wherein one of the CH2 groups may be replaced by CO2 or O;
q=1;
A and B are both phenyl;
X3 and X4 are both H;
Y is CO2 or a single bond;
X or X2 is CF3.
It is believed that these compounds may be characterised by there being intramolecular steric hindrance between the xe2x80x98Xxe2x80x99 group(s) and the adjacent terminal groups of formula I. This in turn may induce a large incipient spontaneous polarisation (Ps) in the molecule which in turn gives rise to a large induced tilt when a field is applied. A large induced tilt usually gives rise to an improved contrast ratio.